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PEER-REVIEWED ARTICLE bioresources.com
Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2128
The Effects of Modifying Peanut Shell Powder with Polyvinyl Alcohol on the Properties of Recycled Polypropylene and Peanut Shell Powder Composites
Nor Fasihah Zaaba, Hanafi Ismail,* and Mariatti Jaafar
The effects of the chemical modification of peanut shell powder (PSP) using polyvinyl alcohol (PVOH) were studied. Both modified and unmodified peanut shell powder were used to prepare recycled polypropylene (RPP) and PSP composites. The effects of various PSP loadings (0 to 40% by weight) on the processing, tensile properties, morphology, Fourier transform infrared (FTIR) spectra, and water uptake properties were examined. Results showed that RPP composites with polyvinyl alcohol-modified PSP had higher values of tensile strength, elongation at break, and tensile modulus, but lower water resistance, than RPP composites with unmodified PSP. FTIR analysis revealed slight changes in band positions and intensities, indicating a distinct interaction between hydroxyl groups of the PSP composites and PVOH. RPP composites with PSP modified with PVOH had better interfacial adhesion between the matrix and the filler than RPP composites with unmodified PSP, as shown by scanning electron microscope (SEM) micrographs.
Keywords: Polyvinyl alcohol; Peanut shell powder; Recycled polypropylene
Contact information: School of Materials and Mineral Resources Engineering, USM Engineering Campus,
Nibong Tebal, Penang, Malaysia; *Corresponding author: [email protected]
INTRODUCTION
Fillers of various composition are widely used in significant quantities in the
plastics industry. Incorporation of filler into the plastic matrix enhances some properties
but detracts from other properties. Fillers could influence the compatibility of the
components of composites (Siriwardena et al. 2001). Natural filler-based composites are
one alternative to the use of synthetic filler-based composites for various applications.
The growing demand for environmentally friendly materials had led to the replacement of
synthetic fillers with natural cellulose-based fillers (Ishak et al. 2010). These kinds of
composite materials are prepared by combining natural, organic fillers derived from
renewable resources with polymers. The polymers can be produced from either petroleum
or from renewable resources. The advantages of natural fillers over their inorganic
counterparts include large availability, low cost, low density, reasonable strength,
reduced energy consumption, and biodegradability (Bledzki and Gassan 1999; Netravali
and Chabba 2003).
Although natural fillers offer many advantages, a major disadvantage of cellulosic
fillers is their highly polar nature, which makes them incompatible with nonpolar
polymers (i.e., PP, PE, and PS). This makes achieving good dispersion and strong
interfacial adhesion between the hydrophilic filler and the hydrophobic polymers difficult
(Kazayawoko et al. 1999). This can lead to poor performance of the final products. Filler
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2129
surface modification, or use of adhesion promoters, is the key to solving this problem.
Surface characteristics such as wetting, adhesion, surface tension, and porosity can be
improved by modifying the filler surface. Many reagents have been used to modify the
cell-wall components to improve adhesion with varying degrees of success; some
examples of these include acid chlorides, isocyanates, aldehydes, alkyl halides,
anhydrides, lactones, and nitriles (Simonsen and Rials 1996). Chemical modifications
may activate hydroxyl groups or introduce new moieties that more effectively interlock
with the matrix (Sreekala et al. 2000). Polyvinyl alcohol (PVOH) is a hydrophilic
polymer with a hydroxyl group on each of its repeating units, which permits the
development of hydrogen bonds of PVOH with the carboxyl and hydroxyl groups of
cellulose fillers.
Peanut is one of the world’s important food crops. The production of peanut
generates large amounts of waste namely peanut shell. Efforts to find utilization of these
waste materials have resulted mostly in low-value or limited applications. In this regard,
peanut shell powder (PSP) seems to be an interesting candidate due to its chemical
composition. Thus, the use of this lignocellulosic filler is of great interest. In this work,
PSP was used as an alternative filler in a recycled polypropylene matrix (Sareena et al.
2011). As in the case of wood, the main components of lignocellulose are cellulose,
hemicellulose, and lignin. Hydrogen bonds between different layers of polysaccharides
make crystalline cellulose resistant to degradation. Cellulose, hemicellulose, and lignin
form structures called microfibrils, which are organized into macrofibrils. Such macro-
fibrils are responsible for the structural stability of plant walls (Jacob et al. 2004). The
contents of cellulose (40.5%), hemicellulose (14.7%), and lignin (26.4%) in peanut shells
have been determined using Official Methods of Analysis of AOAC (15th
Ed.) (Zaaba et
al. 2013).
Previous studies (Sareena et al. 2011) have shown the potential effect of PSP as a
natural filler in natural rubber, but no work has been found in the literature for thermo-
plastic composites. Therefore, in this study PSP was selected as the filler and it was
chemically modified with PVOH in recycled polypropylene (RPP) composites. The
tensile properties, FTIR spectra, SEM micrographs, and water absorption characteristics
of recycled polypropylene (RPP) composites with modified and unmodified PSP at
different filler loadings were compared.
EXPERIMENTAL
Materials Recycled polypropylene (RPP) with a melt flow index of 30 g/10 min was
obtained from Zam Scientific (M) Sdn. Bhd. Peanut shell powder (PSP) was also
supplied by Zam Scientific (M) Sdn. Bhd. The peanut shells were subjected to grinding,
which yielded an average particle diameter of approximately 66.84 μm. PSP was then
dried for 3 h at 70 °C using a vacuum oven before being used for composite fabrication.
99% hydrolyzed polyvinyl alcohol (PVOH) powder, with a molecular weight of 89,000
to 98,000 g/mol and a density of 1.269 g/cm3, was obtained from Sigma Aldrich.
Modification of PSP with PVOH
PVOH powder (6 wt.%) was mixed with PSP. Ethanol was poured into the
mixture. The mixture was then stirred until a homogeneous suspension was formed. The
PEER-REVIEWED ARTICLE bioresources.com
Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2130
suspension was heated at 80 oC for about 15 min to allow the PVOH to dissolve fully and
then stored at room temperature for 24 h, until two distinct layers appeared. The upper
layer was decanted, and the precipitate (PSP modified with PVOH) was dried in an oven
at 70 oC.
Composite Preparation and Processing
Two types of composites were prepared: RPP with unmodified PSP (RPP/PSP)
and RPP with PVOH-modified PSP (RPP/PSPPVOH). They were both prepared using an
internal mixer (Haake Rheomix Mixer, Model R600/610) at a temperature of 180 °C and
speed of 50 rpm to obtain a homogeneous sample. First, the RPP was placed in the mixer
and melted for 4 min. Then, the PSP was added. The composites were mixed for another
8 min until the mixing torque stabilized. The total mixing time was 12 min for all
samples. The processed samples were then compression molded into a 1-mm-thick sheet
by an electrically heated hydraulic press (Kao Tieh Go Tech Compression Machine) at a
temperature of 180 °C. Table 1 shows the formulation characteristics for each of the
composites.
Table 1. Formulation Characteristics of RPP/PSP and RPP/PSPPVOH Composites
Composite RPP (wt.%)
PSP (wt.%)
PSPPVOH
(wt.%)
RPP 100 - -
RPP + 10% PSP 90 10 -
RPP + 20% PSP 80 20 -
RPP + 30% PSP 70 30 -
RPP + 40% PSP 60 40 -
RPP + 10% PSPPVOH 90 - 10
RPP + 20% PSPPVOH 80 - 20
RPP + 30% PSPPVOH 70 - 30
RPP + 40% PSPPVOH 60 - 40
Measurement of Tensile Properties
Tensile tests were carried out with a Universal Testing Machine (Instron 3366)
according to ASTM D638. Dumbbell specimens of 1-mm thickness were cut from the
compression molded sheets with a Wallace die cutter. A crosshead speed of 5 mm/min
was used, and the tests were performed at 25 ± 3 oC. Five specimens were used to obtain
average values for tensile strength, elongation at break, and Young’s modulus.
FTIR Spectroscopy Analysis The functional groups and chemical characteristics of the composites were
obtained by Fourier transform infrared spectroscopy (FTIR, Perkin Elmer System 2000)
with a resolution of 4 cm-1
in a spectral range of 4000 to 550 cm-1
using 32 scans per
sample.
Morphology Evaluation The microstructure of the tensile fractured surfaces of RPP/PSP and RPP/PSPPVOH
composites were compared using a scanning electron microscope (SEM, model ZEISS
Supra 35 VP). The samples were sputter-coated with a thin layer of carbon as it provides
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2131
a good electron-transparent (low atomic number), conductive coating, during the
examination. The images were analyzed to investigate the distribution of natural fillers
and their interactions within the polymeric matrix.
Water Absorption Water uptake measurements were carried out according to ASTM D570. Newly
prepared samples were dried in an oven at 70 °C for 24 h until a constant weight was
attained. The samples were then immersed in distilled water at ambient temperature.
After being immersed for a specific amount of time, the samples were removed from the
water, gently dried with a clean cloth, and immediately weighed to the nearest 0.001 g.
The percentage of water absorption was calculated as follows,
WA (%) = [(M1-Mo)/Mo] X 100 (1)
where Mo and M1 are the dried weight and final weight of the sample, respectively.
RESULTS AND DISCUSSION
Processing Properties The processing torque vs. time curves of RPP/PSPPVOH composites for 0, 10, 20,
30, and 40 wt % filler loadings are shown in Fig. 1(a). All curves have three peaks,
representing the loading of RPP, loading of filler, and stabilization torque, respectively.
RPP was loaded into the mixer at the beginning of the trial. As seen in Fig. 1(a), the
initial peak showed high torque due to resistance to shear by solid RPP against the rotors.
The torque maxima around the 0.5-min mark showed a decrease with filler loading due to
the subsequent decrease in the amount of RPP placed into the mixer (Table 1). At 4 min,
the observed peaks were attributable to the addition of filler into the mixing chamber.
The torque then began to gradually decrease due to a reduction in viscosity (Cao et al.
2011). The stabilization torque could be observed after 8 to 12 min of mixing, when all of
the ingredients of the composite had become mixed homogenously.
Figure 1(b) shows the stabilization torque of RPP/PSP and RPP/PSPPVOH
composites at different filler loadings. Higher filler loadings led to higher stabilization
torque. Similar observations were reported by Siriwardena et al. (2001). This is ascribed
to agglomeration of filler particles, which leads to higher resistance to flow (i.e., higher
torque). This effect became significant as the filler loading approached 40 wt.%. At
similar filler loading levels, the stabilization torque of RPP/PSPPVOH composites was
higher than that of RPP/PSP composites. The torque became more stable when PSPPVOH
particles were homogenized dispersed within the polymer matrix. The chemical similarity
of both PSP and PVOH, both having hydroxyl groups, led to hydrogen bond formation
and a good degree of adhesion between fillers and matrix. Thus, it required high shear
force, resulting in high melt viscosity. Increased viscosity equates to higher resistance to
flow of material within the mixing chamber. Consequently, the stabilization torque also
increases.
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2132
Tensile Properties Figure 2(a)-(c) shows the effects of filler loading on the tensile strengths,
elongations at break, and tensile moduli of RPP/PSP and RPP/PSPPVOH composites. As
can be seen from Fig. 2(a), the RPP/PSPPVOH composites had higher tensile strengths than
RPP/PSP composites. The enhancement in the strength is an indication of a good degree
of adhesion between fillers and matrix. It could be due to the chemical similarity of both
PSP and PVOH, by virtue of both having hydroxyl groups. The existence of such
interaction increases the hydrogen bond formation (see later discussion of IR spectra in
Fig. 6).
Time (min)
0 2 4 6 8 10 12 14
Pro
cess
ing torq
ue (
Nm
)
0
5
10
15
20
25
0 % Filler
10 % Filler
20 % Filler
30 % Filler
40 % Filler
(a)
Filler loading (wt. %)
0 10 20 30 40 50
Sta
biliz
atio
n to
rque
(N
m)
0
2
4
6
8
RPP/PSP
RPP/PSP-PVA
(b)
Fig. 1. (a) The processing torque of RPP/PSPPVOH composites throughout the 12-min mixing, and (b) the stabilization torque of RPP/PSP and RPP/PSPPVOH composites at different filler loadings
PEER-REVIEWED ARTICLE bioresources.com
Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2133
The tensile strength of the composites decreased as filler loading increased. The
reduction of tensile strength could be due to poor adhesion between PSP and the RPP
matrix. The irregular shape of the PSP particles could be another cause of the observed
tensile strength reduction (Fig. 3). As a result of the PSP particles’ irregular shape, their
capability to support stress transmitted from the polymer matrix is rather poor (Cao et al.
2011). The decreasing trend in tensile strength can also be explained by the
agglomeration of filler particles and by the dewetting of the polymer at the interface
(Ismail et al. 2001). The incorporation of filler in RPP/PSP composites also tended to
decrease due to the incompatibility of highly hydrophilic PSP and non-polar hydrophobic
RPP. Because of their differing polarities, the interfacial adhesion between PSP and RPP
was weak, providing sites for failures to initiate and propagate. This is shown by the
SEM micrographs in Figs. 4 and 5.
The effects of filler loading on elongation at break of RPP/PSP and RPP/PSPPVOH
composites are shown in Fig. 2(b). An increase of filler loading in the RPP matrix will
reduce the toughness of the composites and lead to a lower elongation at break (Jacob et
al. 2004). The filler hardened the composites, thus decreasing their ductility. At the same
filler loading, RPP/PSPPVOH composites had higher elongation at break than RPP/PSP
composites. A similar observation was reported by Sreekala et al. (2000). In the cited
study the lower elongation at break of untreated oil palm fibers could be attributed to the
three-dimensionally cross-linked network of cellulose and lignin, which was not present
in treated fibers. The treated fibers had a larger elongation at break.
Figure 2(c) shows the effects of filler loading on the tensile modulus of RPP/PSP
and RPP/PSPPVOH composites. The incorporation of filler was shown to increase the
tensile modulus for both composites. This was due to the inclusion of rigid filler particles
in the soft matrix (Cao et al. 2011). Abdul Khalil et al. (2001) and Ismail et al. (2011)
reported that the incorporation of starch or cellulosic fiber can improve the stiffness of
composite materials. This can be explained by the fact that composite stiffness increases
with higher filler content and higher tensile modulus. At the same filler loading,
RPP/PSPPVOH composites had higher tensile moduli than RPP/PSP composites. This was
due to the strong chemical interaction (hydrogen bonding) between the PSP and PVA,
thus increasing the degree of adhesion between fillers and matrix.
Morphology Scanning electron microscopy (SEM) was used to examine the tensile-fractured
surfaces of RPP/PSP and RPP/PSPPVOH composites. Figures 4 and 5 show SEM
micrographs of RPP/PSP and RPP/PSPPVOH composites at 10, 20, and 40 wt.% filler
loading. Figure 4(a) shows that at low PSP content, the sample had a rough surface and
was deformed in ductile mode. A more porous structure and filler agglomeration were
observed for composites with higher filler content, as shown in Figs. 4(b) and (c). The
porosity of the structure was due to the low filler to matrix adhesion.
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2134
Filler loading (wt. %)
0 10 20 30 40
Te
nsi
le s
tre
ng
th (
MP
a)
0
10
20
30
40
RPP/PSP
RPP/PSP-PVA
(a)
Filler loading (wt. %)
0 10 20 30 40
Elo
ngatio
n a
t bre
ak
(%)
0
2
4
6
8
10
RPP/PSP
RPP/PSP-PVA
(b)
Filler loading (wt. %)
0 10 20 30 40
Ten
sile
mod
ulu
s (M
Pa
)
0
200
400
600
800
1000
1200
1400
1600
1800
RPP/PSP
RPP/PSP-PVA
(c)
Fig. 2. (a) Tensile strength, (b) elongation at break, and (c) tensile modulus of RPP/PSP and RPP/PSPPVOH composites at different filler loadings
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2135
Fig. 3. SEM micrograph showing irregular morphology of PSP
Fig. 4. SEM micrographs of RPP/PSP composites at (a) 10%, (b) 20%, and (c) 40% filler loading
by weight
a
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2136
Fig. 4. SEM micrographs of RPP/PSP composites at (a) 10%, (b) 20%, and (c) 40% filler loading
by weight
This low adhesion made removal of fillers from the composites easy, thus
reducing tensile strength. The pores or voids are flaws that lead to localized stress
concentration during deformation. Finally, premature failure of the composites occurred
at higher filler content, indicating lower tensile strength. Similar results were reported by
Gauthier et al. (1998): as filler loading increases, formation of agglomerates occurs and is
detrimental to mechanical properties (Gauthier et al. 1998).
For composites with treated PSP, a smoother fracture surface can be seen, as in
Fig. 5(a). This indicates that there was better interfacial adhesion between the filler and
matrix in the treated PSP composites. In Fig. 5(b), less filler removal and breakage can be
observed. These differences are due to the modification of PSP with PVOH, which
improves the adhesion of filler to the matrix, thus increasing the tensile strength and
elongation at break of RPP/PSPPVOH composites.
c
Agglomeration
Voids
Filler pulled out
b
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2137
Fig. 5. SEM micrographs of RPP/PSPPVOH composites at (a) 10%, (b) 20%, and (c) 40% filler loading by weight
a
b
c
Filler breakage
Better interfacial adhesion between PSPPVOH and RPP
matrix
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2138
IR Spectroscopic Study FTIR spectroscopy was conducted to determine chemical structural information
for the composites and examine the chemical interactions between the filler and matrix.
Figures 6(a) and (b) show the IR spectra of RPP/PSP and RPP/PSPPVOH composites,
respectively. The spectrum for RPP/PSPPVOH shows some peak changes between 3500
and 3200 cm-1
, which are attributed to O-H stretching bands for hydrogen-bonded
carboxylic acid. A higher wavenumber for the hydroxyl group absorption indicates that
the hydrogen bonding of PSP became stronger when it was modified with PVOH. The
peak at 2917 cm-1
shows C-H stretching of PSPPVOH, while the peaks at 1404 cm-1
and
1375 cm-1
show CH2 and C-O-H bending, respectively. Another additional shoulder peak
was observed at around 1729 cm-1
, and this was identified as the peak for an ester group.
It is interesting to mention that the peak is due to C=O bonds from ester aliphatic
stretching vibrations between hydroxyl groups of PSP and PVOH. Figures 7 show the
proposed interactions between RPP, PSP, and PVOH.
Fig. 6. IR spectra of (a) RPP/PSP and (b) RPP/PSPPVOH composites
Water Absorption Figure 8 shows the water uptake of RPP/PSP composites at different filler
loadings as a function of time. All composites showed a similar pattern of water uptake.
The water uptake of RPP/PSP composites increased with immersion time and increasing
filler loading. The presence of RPP in the composites can dramatically reduce water
absorption due to the hydrophobic characteristics of non-polar RPP. The kinetic
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2139
absorption was fast during the initial stage of the absorption process, then gradually
slowed before finally reaching a plateau.
Fig. 7. Proposed interaction of PSPPVOH with RPP matrix
Natural fillers are permeable to water and have a significant effect on water
absorption. Similar results were reported by Najafi et al. (2006), in which the percentage
of water absorption increased with higher filler content. This is due to the highly
hydrophilic nature of lignocellulosic filler. The free hydroxyl groups come in contact
with water through hydrogen bonding, resulting in water uptake and weight gain in
composites. In addition, higher filler loading resulted in more pores within composites,
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2140
thus increasing water accumulation at the interface between the filler and the matrix
(Jacob et al. 2005).
Figure 9 shows the equilibrium of water absorption for both types of composites
after 30 days. The water absorption of RPP/PSPPVOH composites is higher than for the
RPP/PSP composites. This is due to the incorporation of PVOH into the RPP/PSPPVOH
composites. PVOH is a hydrophilic polymer that absorbs water. The IR spectra in Fig. 4
show an increase in the intensity of -OH stretching bands, which confirms the interaction
between PVOH and water.
Time (days)
0 5 10 15 20 25 30
Wate
r upta
ke (
%)
0
2
4
6
8
0 % Filler
10 % Filler
20 % Filler
30 % Filler
40 % Filler
Fig. 8. Water uptake over 30 days of RPP/PSP composites at different filler loading
Filler loading (wt. %)
0 10 20 30 40 50
Wat
er u
ptak
e (%
)
0
2
4
6
8
RPP/PSP
RPP/PSP-PVA
Fig. 9. Equilibrium water uptake of RPP/PSP and RPP/PSPPVOH composites at 30 days
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Zaaba et al. (2014). “Peanut shell PP composite,” BioResources 9(2), 2128-2142. 2141
CONCLUSIONS
1. Incorporation of PVOH into RPP/PSP composites increased the stabilization torque,
tensile strength, elongation at break, and tensile modulus.
2. Incorporation of PVOH into RPP/PSP composites decreased the water resistance.
3. A smooth fractured surface can be observed in SEM micrographs for composites with
treated PSP, indicating a better interfacial adhesion between filler and matrix than for
unmodified PSP composites. Composites without PSP treatment had rough surfaces,
porous structures, and filler agglomeration and deformed in ductile mode. These
observations indicated that low filler-matrix adhesion led to easy removal of fillers
from the composites.
4. FTIR analysis revealed slight changes in band positions and intensities, indicating a
distinct interaction between the hydroxyl group of PSP and PVOH.
5. The water uptake of RPP/PSP composites increased with immersion time and
increasing filler loading. Water absorption of RPP/PSPPVOH composites was higher
than that of RPP/PSP composites.
ACKNOWLEDGMENTS
The authors would like to acknowledge the financial support provided by a
Research University Grant from Universiti Sains Malaysia (USM) and MyBrain15 from
the Ministry of Education of Malaysia.
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Article submitted: November 19, 2013; Peer review completed: January 29, 2014;
Revised version received and accepted: February 17, 2014; Published: February 25, 2014.